For example, as shown in FIG. 2, a multilayer ceramic capacitor, one of representative ceramic electronic devices, has a ceramic element 54 and two external electrodes 55a and 55b provided on two end portions thereof. The ceramic element 54 has internal electrodes (such as Ni electrodes) 51 which are disposed to face each other with ceramic layers 52 interposed therebetween and which are alternately extended to two end surfaces 53a and 53b located in opposite directions, so that the two external electrodes 55a and 55b are electrically connected to the internal electrodes 51.
In addition, the external electrodes 55a and 55b are formed, for example, by the steps of applying a conductive paste (Cu paste in this case) to the ceramic element 54, then baking the conductive paste to form Cu-baked electrode layers 56a and 56b, forming Ni plating layers 57a and 57b on the Cu-baked electrode layers 56a and 56b to prevent dissolution by solder, and further forming Sn plating layers 58a and 58b on the Ni plating layers 57a and 57b in order to improve solder wettability, and hence the external electrodes 55a and 55b have a three-layered structure composed of the Cu-baked electrode layers 56a and 56b used as an underlayer, the Ni plating layers 57a and 57b used as an interlayer, and the Sn plating layers 58a and 58b used as an upper layer (surface layer), respectively.
Although the Ni plating layers 57a and 57b are to be used as an interlayer function to prevent entry of a Sn plating solution and moisture of external environment, since minute voids are inherently present in the Ni plating layers 57a and 57b, the function of preventing the entry of a plating solution and moisture is not satisfactory.
Accordingly, in order to prevent the entry of a plating solution and moisture, the Cu-baked electrode layer must be formed in a dense state by baking. However, in a baking step in which the Cu-baked electrode layer is densified by baking, glass moves up to the surface of the Cu-baked electrode layer and precipitates thereon, thereby, for example, causing a plating defect in a subsequent step of forming the Ni plating layers 57a and 57b and/or causing solder popping in a mounting step (soldering step) due to degradation in denseness of the Ni plating layer. As described above, the defects caused by so called “glass scum” disadvantageously occur.
Accordingly, in order to solve the above problems, a method has been proposed (for example, in Patent Document 1) to obtain a baked electrode which can be densified and which has good plating properties by optimizing an electrode material.
In this method, a conductive paste is used which contains a powdered metal primarily composed of a powdered Cu including a flake-shaped Cu powder having a specific surface area, a long diameter, and a thickness, which meet respective specifications, and a spherical-shaped Cu powder at a predetermined ratio; a glass powder; a binder; and an organic solvent, and external electrodes are formed by applying this conductive paste on predetermined regions of a ceramic element, followed by baking.
In addition, according to this method, it is described that external electrodes can be formed which have an appropriate denseness and which are able to prevent precipitation of a glass component (binding agent) on the surface and the entry of a plating solution inside the ceramic element.
However, since this method is liable to be influenced by the changes in electrode material and process, it is not easy to ensure sufficient reliability, and in addition to that, there has been a problem in that higher-level requirements in the future cannot be easily satisfied.
In addition, as another conventional technique, an electronic device has been proposed (for example, in Patent Document 2) in which as shown in FIGS. 3(a) and (b), as external electrodes 64 provided for an electronic device main body 65 having internal electrodes 61 disposed therein, external electrodes are formed each having a four-layered structure composed of a Cu-baked electrode layer 66, a Cu plating layer 67, a Ni plating layer 68, and a noble-metal plating layer 69.
That is, in this electronic device, the external electrodes 64 each have a four-layered structure composed of a first external electrode layer (Cu-baked electrode layer 66) formed by baking a past containing powdered Cu or a powdered Cu alloy, a second external electrode layer (Cu plating layer 67) of a Cu plating film formed on the first external electrode layer, a third external electrode layer (Ni plating layer 68) of a Ni plating film formed on the second external electrode layer, and a fourth external electrode layer (noble-metal plating layer 69) of a noble-metal plating film formed on the third external electrode layer.
However, in the case of this Patent Document 2, since minute voids are generally present in the Cu plating layer 67, the entry of a plating solution and moisture of external environment cannot be always sufficiently prevented, and as a result, there has been a problem of inferior reliability.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-172383    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-55679